Allelotoxicity of Greenhouse Substrates after Vegetable Cultivation.

Crop exposure to stress during cultivation is known to reduce the yield and to cause the release of allelotoxins from plants into soil. It was assumed that allelotoxin release may considerably affect the vegetable growth in greenhouses and that a decrease in the allelotoxin concentration in greenhouse substrates may improve the plant growth. To verify the assumptions, allelotoxicity and microbial contents were determined in greenhouse substrates in which cucumber, tomato, and pepper plants grew well or poorly. The allelotoxin content was found to be higher and the prokaryote content, lower in the substrates of poorly growing plants. The finding confirmed the assumption that allelotoxins significantly influence the cultivation of vegetables in greenhouses. Treating the plant root zone with humate solutions having a high allelotoxin absorption capacity appreciably improved the cucumber plant growth and was assumed to provide a promising means to increase the vegetable yields in greenhouses.

Plant Stress and Allelotoxicity of Soils.

Stress exposures during vegetation are known to reduce the yield in crops, but the intensity and duration of stress is rather difficult to determine from the crop loss. Allelotoxins are released from plants into soil on exposure to stress factors. Soil allelotoxicity after vegetation was assumed to provide a diagnostic parameter that characterizes the total stress effect on crops. To verify the assumption, changes in soil allelotoxicity were studied after vegetation of three winter wheat cultivars fed with different amounts of nitrogen fertilizers. The cultivars Nemchinovskaya 85, Moskovskaya 39, and Lutescens 982/08 were found to differently respond to fertilizing. Nemchinovskaya 85 and Lutescens 982/08 showed lower stress responses at higher doses of nitrogen fertilizers, while their responses were appreciable when soil nitrogen was lacking. In the case of Moskovskaya 39, higher doses of nitrogen fertilizers caused an adverse reaction of increased allelotoxin release. Stepwise nitrogen fertilization was concluded to be necessary for preventing delayed plant development in the case of this cultivar. The results confirmed that soil allelotoxicity measured growing plants can serve as an indicator of stress caused by the growth conditions and use of various agricultural techniques.

State of Soil Gels in Soddy-Podzolic Soil Samples Prepared for Viscosimetry by Different Methods.

As is well known from the mid-20th century, films of organomineral gels cover and bind soil particles in soils. Soil contact with water has been shown to lead to water absorption by gels and gel swelling. The change of gel properties in soils should manifest itself in a change in the viscosity of soil pastes. A vibrating viscometer was proposed to use to determine the viscosity of soil pastes. The physical meaning laid down in Einstein's formula was used to interpret the results. This made it possible to assess the degree of gel swelling by the amount of water that remained capable of moving independently of soil particles, that is, was not included in soil gels (free water). The effect on the degree of swelling of soil gels was studied for (1) the moisture content in soil samples used to prepare soil pastes, (2) the time after adding water during the preparation of soil samples used subsequently to obtain soil pastes, and (3) the sample preparation of soil samples (initial, autoclaved, or dried to air-dry and absolutely dry states and re-moistened) used to obtain soil pastes. Experiments showed that (1) the degree of swelling of soil gels increased with the increasing moisture content in soil samples, (2) a longer time of interaction of soil samples with water led to a greater degree of swelling of soil gels, and (3) different degrees of swelling of soil gels were observed in pastes prepared from soil samples that had the same moisture content, but differed in sample preparation protocol.

Allelotoxicity and Water Stability of Sod-Podzolic Soil after Growing Wheat on It.

It is well-known that soil allelotoxicity and the water stability of soil structure are determined by the entry of plant residues into the soil, but the relationship of these soil properties has not been investigated. Soil samples from the fields of the Federal Research Center "Nemchinovka" after growing 25 cultivars of spring and winter wheat on plots with sod-podzolic soil are selected in this study. The effect of the soil's allelotoxicity of plot samples on the germination of spring wheat seed of the Liza cultivar is studied by the biotesting method. The water stability of the soil structure is determined for the same samples. The existence of a sigmoid dependence between the allelotoxicity of soil samples and their water stability with a correlation greater than 95% is established. The presence of this dependence is apparently related to the increase in the soil's water stability with an increase in the amount of fresh plant residues entering them, which is known from the literature, and the transition of the allelotoxins' effects from stimulation to suppression with a growth in their concentration in soils.

On the Sorption-Stimulating Preparations Influence on Seed Germination.

The influence of various types of impacts on soil samples on the germination of seeds and the development of seedlings in comparison with sand (allelotoxicity) was studied. It is established that various types of soil sample preparation can significantly affect their allelotoxicity. It is shown that initial soil stimulated seedling development (+23%). Drying to air-dry state with further wetting increased the allelotoxicity to -27%. Autoclaving of samples (148°C, 4.5 atm) increased their allelotoxicity to -77%. The data obtained suggest that allelotoxins can exist in the soil in active and fixed forms and pass from the fixed form to the active form under various types of impacts on the soil. The study of the influence of the allelotoxicity of soil samples on the application efficiency of sorption-stimulating preparation (SSP), used for the pre-sowing treatment of spring wheat seeds, shows that the application effect of SSP monotonically decreases when allelotoxicity increases. The calculations show that the amount of sorbent used to protect seeds from allelotoxins is 3-4 orders of magnitude lower than the amount that can fix allelotoxins contained in soils. It follows from above that the development of seed seedlings is affected only by a small part of allelotoxins present in soils in the active state.

On the Nature of the Influence of Some Mycelial Actinobacteria on the Spring Wheat Seeds Germination in Soils.

The effect of pre-sowing treatment of spring wheat seeds with suspensions of various actinomycete strains has been studied. It has been found that 4 out of 18 strains studied had a stimulating effect on seed germination in soils and plant development. The stimulation of seeds by these strains was not observed during seed germination in sand. It has been suggested that the mechanism of acceleration of seed development by actinomycetes could be based not only on the release of stimulating biologically active substances, but also on the consumption of allelotoxins by these microorganisms. The test carried out to study the possibility of actinomycete growth on a medium containing one of the most common allelotoxins, coumarin, has shown that actinomycetes promoting seed germination develop on this medium more intensively compared to strains that do not affect or inhibit the germination of spring wheat seeds. This confirms the proposed hypothesis that actinomycetes can stimulate seed germination through consumption of soil allelotoxins.

The Nature of the Increased Efficiency of Sorption-Stimulating Preparations Containing Non-Ionic Surfactants for Pre-Sowing Seed Treatment.

The effect of the introduction of a non-ionogenic surfactant Polysorbate 20 into a sorption preparation (CB-H-BYA) on the structure of sorbent layers formed on the surface of spring wheat seeds during their pre-sowing treatment has been studied using electron microscopy. According to the results, an increase in the efficiency of sorption preparations containing Polysorbate 20 is based on an intensification of the bentonite aggregate disintegration into individual montmorillonite particles and a formation of more dense protective sorption layer providing a better protection of seeds against allelotoxins on the seed surface. The introduction of non-ionogenic surfactants into the preparation increases a sorption capacity of a bentonite-humus complex that results in a decreased gibberellin activity in a solution. Therefore, to achieve the maximum physiological activity of gibberellin in a preparation solution, it is necessary to increase its concentration from 100 to 300 mg/L. As a result, the stimulating effect increases from 36 to 55%.

Plant Growth Hormones Increase the Stimulation Efficiency of Seedlings Development for Spring Wheat Seeds upon Pre-sowing Treatment.

The influence of pre-sowing treatment of spring wheat seeds with combined use of plant growth hormones and sorption preparations based on bentonite-humate mixtures on seeds germination and their development in soils was studied. In some cases, the combined use of plant growth hormones and the sorption preparation (CB-H-BYA) that can decrease the intake of allelotoxins from soil to seeds allows noticeably increasing the efficiency of plant growth hormones used for pre-sowing treatment. The inclusion of cytokinins (6-benzylaminopurine, kinetin, and forchlorophenuron) into the sorption preparation (CB-H-BYA) had markedly different effects on seeds germination. The addition of Polysorbate 20 to the sorption preparation (CB-H-BYA) leads to an increase in the effectiveness of its action on seed germination.

Stimulation of Seed Germination by Humic Substances: On the Nature of the Phenomenon.

The wheat seed germination and seedling growth, as well as the effect of humic substances on seed development, were compared using soils and sand as substrates. Russian Plain soils were found to considerably inhibit seed germination. Pre-sowing treatment with humic preparations resulted in a significant stimulation effect on seeds sowed into sod-podzolic soil, but not into sand. Considering a well-known phenomenon of soil allelelotoxicity, we have suggested that stimulation of seed development by humic substances is a result of seed protection by them against the allelotoxin inhibitory effect.